The heart cycle provides both electrical and audio (or phono) signals containing invaluable diagnostic information for the practicing physician. The electrical system of the heart includes a pacemaker or electrical generating source near the right atrium known as the sinoatrial (SA) node. When the SA node generates an electrical pulse, that pulse is transmitted to both atria causing them to contract to force blood through the atrio-ventricular valves and into the ventricles. The generated wave pulse then reaches a atrio-ventricular (A/V) node, which slows down the speed of the impulse to allow the blood to flow from the atria to the respective ventricles. After a time delay of about 100 milliseconds, this impulse is then transmitted to the ventricles through the Purkinge fibers, effecting contraction of the ventricles. Thereafter, the ventricles relax. The above named electrical sequence occurs for each beat of the normal heart.
The aforementioned transmitted pulse effects the electrical depolarization of the heart muscle fibers. Repolarization of the heart muscle occurs upon relaxation. Depolarization and repolarization are electrically measurable phenomena and form the basis for the well known electrocardiogram (ECG) measurement of heart activity. The ECG analog waveform associated with the described heart activity includes a number of medically significant segments. In the case of a normal "at rest" heartbeat, there first occurs a "P wave". This "P wave" comprises a small rise and drop in the analog signal amplitude. This wave is representative of the depolarization of the atria and is immediately followed by a generally short level signal portion which terminates in a drop in signal amplitude. This drop is referred to as the "Q wave". The portion of the ECG between the end of the "P wave" and the beginning of the "Q wave" is called "PQ interval" or "PQ segment". Physiologically, this interval represents a pause in the heart sequence and corresponds to the flow of blood from the atria into the ventricles.
Following the "Q wave" there is a rapid rise in the signal voltage to maximum amplitude known as the "R wave". A corresponding sharp drop in voltage amplitude to below the level of the "PQ interval" followed by a rise back to such level is characterized as the "S wave". The "S wave" is then followed first by a level segment which terminates in a further rise and fall in amplitude known as the "T wave". The point where the "S wave" terminates in the beginning of the level segment is known as the "J point", and the level segment between the end of the "S wave" and the start of the "T wave" is known as the "ST segment". The Q, R, and S waves are referred to as the "QRS complex" and represent ventricular contraction (depolarization). The "T wave" represents repolarization of the ventricles. After the "T wave" there is a brief, further level segment before the commencement of the "P wave" of the next heart beat. This latter segment is referred to as the "TP segment". Thus, the various heart activities are well defined in terms of the electrical impulses. The various ECG segments illustrated with respect to a normal "at rest" heart are shown in the electrocardiogram of FIG. 1.
Just as the heart cycle is characterized by the above-referenced sequence of electrical events, it is similarly characterized by a sequence of acoustical events. Normally, the most significant of these events are denominated "Heart Sound I" (HS I) and "Heart Sound II" (HS II) although additional heart sounds of significance may be identified. The identification of information as a particular heart sound is critical for diagnostic purposes. Although the sounds are defined by different frequency bands (HS I, for instance, is in the medium high and high frequency audio range of 100 to 1000 cycles per second while HS II is in the medium low frequency range of 50 to 100 cycles per second), the physician, especially one having a hearing impediment, is likely to have significant difficulty in direct analysis. Objective identification of the particular sound, however, is suggested by the following known relationships: (1) the first group of vibrations of heart sound one HS I, coincides with either the "R wave" or the RS slope of the ECG, (2) the aortic components of heart sound two HS II, coincide with the end of the "T wave" of the ECG, (3) heart sound four, HS IV, (a diphasic or triphasic slow wave indicating presystole) falls at the end of the "P wave" and always precedes the Q wave and (4) abnormal vibrations (including opening snap, heart sound three) have no definite coincidence with the ECG. These relationships can be seen by a comparison of the ECG data of FIG. 1 with the (simultaneously generated) low frequency sound data of FIG. 2. Abnormal first and second sounds can indicate hypertension, mitral stenosis, bundle-branch block, myocarditis, aortic insufficiency, thyroid disease, diastolic and systolic overload and many other diseases. Abnormal third and fourth heart sounds are indicative of various hemodynamic disturbances such as heart murmurs. Thus, the accurate identification of heart sounds provides a diagnostic tool of broad range.
Presently two methods are commonly used for extraction of the useful data contained in heart sounds; (1) auscultation with the aid of a stethoscope and (2) phonocardiography. By far the most widely used of these methods is auscultation. Such analysis, however, is hampered by the limitations of the human ear. The normal ear is incapable of detecting many of the sounds produced by the action of the heart, resulting in a loss of information of diagnostic value. Additionally, the common stethoscope does not identify that portion of the heart cycle which is generating a particular sound. Such indeterminancy often results in the mis-identification of heart sound data and negates, or worse yet, results in inaccurate diagnosis of, the raw sound data.
Recognition of the useful data contained in heart sounds and of the above-described relationships has led to the development of phonocardiography. This art attempts to circumvent the inherent subjective difficulty in interpretation of heart sounds by the visual display of simultaneously generated sound and ECG data in the manner of FIGS. 1 and 2.
Present day phonocardiography is practiced by means of rather bulky systems containing both ECG and audio channels to monitor the heart's action. Typically, the data generated are displayed on strip chart recorders which provide the physician, generally a cardiologist, with a permanent, objective record of heart activity that utilizes the known relationship of heart sounds and ECG. The two "readings" are displayed on side-by-side graphs generated by the coaction of voltage-responsive styluses with a motor-driven roll of graph paper and diagnosis is made on the basis of the known temporal relationship of heart sounds and ECG. The styluses must be allowed freedom of movement which, along with the space required to house the chart drive motor and other mechanical components, serves to contribute to the bulkiness of the device. Additionally, the motor places a large power-consumption requirement upon the present-day phonocardiograph.
The phono data of the current phonocardiogram is displayed directly by the strip chart recorder when the low frequency data (approximately 80-100 Hz) are selected. Mid and high frequency data of the phonocardiogram are generated by the amplitude modulation of a carrier frequency. The carrier is necessarily of very low frequency with respect to a variety of significant heart sounds for compatibility with the electromechanical stylus. Preejection click, for example, is characterized by a fast, short wave (300-1000 Hz) which cannot be delineated effectively upon the 85 to 100 Hz carrier frequency generally employed. This disparity in frequencies occasioned by the electromechanical tracing of extremely high frequencies has led to the necessary application of envelope detection methods to the heart sounds prior to carrier modulation. These methods, which involve the smoothing of the high frequency waveform to accentuate the prominent components thereof, necessarily involves the use of filters and the selection of appropriate system time constants (attack, decay times) and such choices, in turn, incorporate tradeoffs whereby detail is sacrificed for the "overall picture".
In addition, the taking of a phonocardiograph by means of bulky, present day apparatus entails a complex patient "hook up" process that includes the attachment of a sensor having a diameter of approximately two (2) inches to the patient to detect the heart sounds and transform them into stylus-influencing electrical signals. The sensor, which attaches to the chest wall, obscures the area around the heart and prevents the physician from listening to the heart sounds as they are being recorded and displayed. To the general physician, unfamiliar with visual display of heart sound data, the absence of audio sensing of the heart sounds is often an uncomfortable experience until practice "weans" him away from his aural crutch.